The goal of this proposal is to understand the mechanism of voltage-dependent opening and closing of ion channels. These voltage-dependent channels have cationic helices, voltage sensors that move within the membrane in response to changes in membrane potential. This movement drives the opening and closing of the channel. A new model of voltage sensing has emerged from studies of a potassium ion channel know as KvAP found in a archaebacterium. This new model contradicts the conventional model of voltage-dependent sensing. The first aim is to use fluorescence spectroscopic assays to study the orientation and topology of the isolated voltage sensor domain of the KvAP protein in vesicles as the transmembrane potential is modulated. The second aim is to study how primary sequence of the voltage sensor domain effects voltage sensing. To study this question, mutants of the voltage sensor will be studied in liposomes with fluorescence assays mentioned above. And, the third aim is to construct a modified version of the voltage sensor domain of KvAP that has voltage-dependent fluorescence properties. This novel protein could be used as an optical sensor of electrical activity in cells.